Particle filters remove soot particles from the lean exhaust gas of combustion engines, thereby preventing their emission into the atmosphere. Various filter concepts may be employed to accomplish these goals, including but not limited to wall flow filters, filters of ceramic fibers or of ceramic or metallic foams, and filters of wire meshes. These technologies have proven extremely successful at accomplishing their designed tasks. In fact, they can attain filtration levels greater than 95%. Unfortunately, while they are effective at filtering, the filters themselves are not easily regenerated.
Carbon soot, which is one of the major substances trapped on particle filters, burns spontaneously only at temperatures of about 600° C. or greater. But modern diesel engines, generally reach such temperatures only at full load. Therefore, under normal operating conditions, the filters can become plugged after a limited operation time unless additional supportive measures are used to oxidize the soot particles that are separated from the exhaust gas and build up on the particle filters.
Two types of supportive measures may be employed to regenerate the filters: active and passive. With active measures, the temperature of the filter is raised, by, for example, electrical heating, above the temperature needed for oxidation of the soot. Such measures are linked to increased fuel consumption. With passive acting systems, for example, the ignition temperature of the soot is reduced by use of organometallic fuel additives such as ferrocene or by a catalytic coating on the filter.
A number of solutions using different active and passive measures have been offered. For example, German Patent DE 31 41 713 A1 describes a coating containing silver vanadate as the active substance, which reduces the soot ignition temperature. A further development of that invention is described in DE 32 32 729 C2. According to the latter patent, the coating that reduces the ignition temperature can contain lithium pentoxide, vanadium pentoxide with an alkali metal oxide, a vanadate, a perrhenate, or a combination of those substances as the active substance.
DE 34 07 172 describes a means for cleaning oxidizable solid, liquid and gaseous pollutants from diesel engine exhaust gas. For this purpose, the means comprises filter elements arranged successively, either in immediate succession or with spaces between them, in a housing. At least one filter element A, which carries the catalyst that reduces the ignition temperature of the soot and promotes its combustion, and at least one filter element B, which carries the catalyst promoting burning of gaseous pollutants, alternate repeatedly.
Koberstein et al., “Einsatz von Abgashachbehandlungseinrichtungen” [“Use of Exhaust Gas Treatment Systems”] (VDI [Society of German Engineers] Report Number 559, VDI-Verlag [VDI Press] 1985, 275-296) describes a wall flow filter having a combined coating with an ignition catalyst on the channel walls at the gas inlet side and an oxidation catalyst on the gas exit side. There the function of the oxidation catalyst is to oxidize the hydrocarbons and the carbon monoxide released during the filter regeneration, thus making them harmless.
U.S. Pat. No. 4,510,265 describes a self-cleaning diesel particle filter. In that reference, the filter has a catalyst mixture of a metal of the platinum group and silver vanadate. Presence of the catalyst mixture reduces the ignition temperature of the diesel particles.
U.S. Pat. No. 4,849,399 likewise describes a catalyst composition to reduce the ignition temperature of diesel soot. In that reference, the composition contains sulfur-resistant inorganic oxides selected from the group consisting of titanium oxide, zirconium oxide, silicon dioxide, aluminum silicate and aluminum oxide and catalytically active components selected from the group consisting of platinum, palladium and rhodium deposited on the oxide.
According to U.S. Pat. No. 5,100,632, the ignition temperature of diesel soot can be reduced with a catalyst composition containing a platinum group metal and an alkaline earth metal. A catalyst composition of magnesium oxide and platinum and/or rhodium is particularly recommended.
U.S. Pat. No. 5,758,496 describes a particulate and exhaust gas cleaning system containing a particle filter, the porous walls of which are directly coated with a catalytically active metal to oxidize carbon monoxide and unburned hydrocarbons. An additive is added to the fuel to reduce the ignition temperature of the diesel soot deposited on the filter. This additive comprises an organometallic compound in a liquid carrier medium. In particular, it involves organometallic compounds of copper octoate, nickel octoate, or cerium octoate.
U.S. Pat. No. 5,792,436 describes a process for removal of nitrogen oxides and sulfur oxides from the lean exhaust gas of combustion engines. For this purpose, the exhaust gas is passed over a catalyzed trap that contains a combination of a material that absorbs nitrogen oxides and sulfur oxides, and an oxidation catalyst. The absorbing material can be regenerated by raising the temperature of the trap. For that purpose, burnable components are added to the exhaust gas flow during the regeneration phase. Those components are burned at the oxidation catalyst and raise the temperature of the trap to the temperature at which nitrogen oxides and sulfur oxides desorb. Suitable absorber materials are oxides, carbonates, or hydroxides of magnesium, calcium, strontium, barium and lanthanum, as well as oxides of cerium or praseodymium and oxides of elements with atomic numbers of 22 to 29. The oxidation catalyst comprises at least one platinum group metal. The absorbing material and oxidation catalyst are applied as a coating, for example, on a honeycomb structure with parallel open channels or on spherical or tablet-shaped supports arranged in a packing.
U.S. Pat. No. 6,023,928 describes a process for simultaneous reduction of soot particles, unburned hydrocarbons, and carbon monoxide in diesel engine exhaust gas. The process described in that reference utilizes a particle filter catalyzed with platinum in combination with a cerium-containing fuel additive to reduce the ignition temperature of the soot.
Reduction of the soot ignition temperature by a soot ignition coating or by a fuel additive generally cannot assure regeneration of a filter at low engine load. Consequently, frequently a combination of active and passive measures is now used.
The combination of an oxidation catalyst with a particle filter has proven particularly useful. In this type of system, the oxidation catalyst is placed ahead of the particle filter. Because of post-injection or other measures applied to the engine, unburned fuel and carbon monoxide arrive at the oxidation catalyst, where they are converted catalytically to carbon dioxide and water. The heat released in the reaction heats the exhaust gas, and thus also the subsequent particle filter. For example, British Patent 2 134 407 A describes one such system. The amount of post-injection of fuel can be reduced by combination with a catalytic filter coating that reduces the soot ignition temperature or by fuel additives, and the filter can be regenerated at any operating point of the engine.
EP 0 341 832 B1 takes a different path. It describes a process for treating the exhaust gas from heavy trucks. In that reference, the exhaust gas is first passed, without filtering, over an oxidation catalyst so as to oxidize the nitric oxide that it contains to nitrogen dioxide. The exhaust gas containing nitrogen dioxide is then used to burn particles deposited on a subsequent filter, the amount of the nitrogen dioxide being enough to burn the particles deposited on the filter at a temperature of less than 400° C. This system is said to allow continuous regeneration of the particle filter without requiring periodic post-injection of fuel to increase the exhaust gas temperature.
EP 0 835 684 A2 describes a process for treating exhaust gases of light trucks and automobiles. According to this process, the exhaust gas is passed over two catalysts in succession. The first catalyst oxidizes the nitric oxide in the exhaust gas to nitrogen dioxide, which in turn oxidizes to CO2 the soot particles that have deposited on the second catalyst.
The processes described in the last two patents assume a high proportion of nitrogen oxides in the untreated diesel exhaust gas. Unfortunately, it is not usual for that proportion to be sufficient.
In a press release of Apr. 15, 1999, PSA Peugeot Citroën presented a particle filter system for diesel engines with periodic regeneration of the particle filter by burning of the soot particles deposited on the filter. The soot particles deposited on the filter burn in the presence of oxygen only at a temperature of 550° C. Several measures are taken to assure regeneration of the particle filter even during operation of the diesel engine with exhaust gas temperatures of only 150° C. (during trips in the city, for example). For one, the exhaust gas temperature is raised to 450° C. by active measures. Second, a cerium-containing additive that reduces the natural combustion temperature of the soot particles to 450° C. is added to the fuel. Fuel is injected into the cylinders during the expansion phase to raise the exhaust gas temperature. This process is referred to as “post-injection.” The afterburning due to that process raises the exhaust gas temperature by 200 to 250° C. There is also further afterburning of unburned hydrocarbons resulting from the post-injection on an oxidation catalyst placed ahead of the filter, which raises further the exhaust gas temperature by another 100° C.
A further critical advantage of the fuel additive is the accelerated filter regeneration. However, it has not yet been possible to assure the post-injection required to initiate the filter regeneration at every operating point. Acceleration of soot combustion, therefore, reduces the occurrence of incomplete filter regeneration. The time during which the post-injection must be maintained can be minimized, which clearly reduces the danger that during regeneration the vehicle may arrive at an operating point for which post-injection is not suitable.
The known processes and exhaust gas systems that add an additive to the fuel to reduce the soot ignition temperature have the disadvantage that after regeneration of the filter, the additive accumulates in the filter as an ash, such as cerium ash. Ash also appears from combustion of lubricating oil carried by the exhaust gas (oil ash). The cerium ash makes up about two thirds of the total ash. With the oil ash, it forms a powdery flocculent composition that remains as a residue in the filter after burning of the soot. After a certain period of operation, depending on the size and oil consumption of the engine, several hundred grams of ash can collect in the filter. That ash substantially increases the exhaust gas backpressure, resulting in continuously increasing fuel consumption. Therefore, the ash is usually removed from the filter by washing with water after a long period of operation, such as after about 80,000 km.
A further disadvantage of this system, and those systems that operate with uncoated filters, is the high carbon monoxide emissions that occur during the spontaneous or active filter regeneration. The soot collected in the filter, at a loading of up to 8 grams of soot per liter of filter volume burns within a few minutes, which results in incomplete oxidation of the soot particles and in substantial CO emissions.
In light of the above-described problems, the present invention is directed to a particle filter that, by means of a catalytic coating, is able to reduce the soot ignition temperature, accelerate soot burning, and so reduce the energy requirement for regeneration of the soot filter and the danger of interruption of the regeneration. The present invention is also directed to the reduction of CO emissions during active filter regeneration, while extending the interval between two washings of the filter to remove accumulated ash. Further, the present invention is also directed to a process for accelerated combustion on the particle filter of soot particles collected from the lean exhaust gas of a diesel engine.